1. Field of the Invention
The present invention relates to a silicon-containing vinyl copolymer, and more particularly to a resist composition containing the silicon-containing vinyl copolymer. The silicon-containing vinyl copolymer is suitable for use as a top layer resist in a bilayer resist system.
2. Description of the Prior Art
With increasing integration of semiconductor devices, there is a heightened need to form finer patterns in photolithography processes. Both 248 nm and 193 nm photolithography technologies are necessitated in fabricating microelectronic devices, and many efforts have been made to develop a suitable photoresist material for operating at such short wavelengths.
When a single layer resist is used in the short wavelength photolithography, even if an anti-reflection resist layer is applied, the depth of focus (DOF) will be decreased, and the stability control capacity of the process will become worse. Therefore, a bilayer resist has been developed to improve the resolution and the stability control capacity of the process. The bilayer resist includes a thicker underlayer made of a resin, which can planarize the substrate and decrease the reflection, and a thinner silicon-containing top layer made of a silicon-containing polymer, which is photosensitive and has good resistance toward oxygen plasma etching.
Kim et al. have disclosed a silicon-containing methacrylate-based polymer for use as a top layer of a bilayer resist system, in which 2-trimethylsilyl-2-propyl is introduced as an acid labile protective group into the methacrylate unit [Polymer, 40, 1617-1621 (1999)]. Upon exposure and post exposure bake (PEB), the silicon-containing labile group will be removed, thus resulting in a different solubility between the exposed and unexposed regions. Therefore, the resist can be developed by a conventional developer. In addition, since the underlayer resin and the top layer resist have different silicon contents, the pattern of the top layer can be transferred to the underlayer by dry etching using oxygen reactive ion etching (RIE).
Kim et al. have further disclosed a silicon-containing norbornene-based polymer, which is poly(5-((2-trimethylsilyl-2-propyl)oxycarbonyl)-norbornene-co-maleic anhydride) [SPIE, 3999, 1079-1087 (2000)]. In this norbornene-based polymer, 2-trimethylsilyl-2-propyl ester is introduced as an acid labile protective group into the norbornene unit.
Schaedeli et al. in WO 99/42903 have disclosed a methacrylate-based terpolymer, in which tris(trimethylsiloxy)silylpropyl is introduced into the methacrylate unit.
However, the conventional bilayer resist suffers from the disadvantages that the silicon-containing polymer has inferior film forming properties and the film cracks easily. Further, the low glass transition temperature makes that the post exposure baking temperature can not be too high, which adversely affects the diffusion rate of the photoacid. In addition, the adhesion between the top photosensitive layer and the resin underlayer is inferior. Therefore, there is still a need to develop an improved silicon-containing polymer for a bilayer resist system.
Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a novel silicon-containing copolymer having a high glass transition temperature and a better polarity.
Another object of the present invention is to provide a resist composition having good adhesion to a resin underlayer for use in a bilayer resist system.
To achieve the above objects, the silicon-containing copolymer of the present invention is a silicon-containing vinyl copolymer, which includes repeating units represented by the following formulae 
wherein
m is 1 or 2;
D is an acid-labile protective group, which decomposes in the presence of an acid, so as to make the silicon-containing vinyl copolymer alkali-soluble;
R1 is selected from the group consisting of hydrogen and C1-8 linear and branched alkyl; and
R2 is selected from the group consisting of 
wherein n is 0 or an integer of from 1 to 6, and R3 is C1-8 linear or branched alkyl.
The silicon-containing vinyl copolymer of the present invention can be prepared from the corresponding monomers by any suitable conventional polymerization methods, such as, by free radical, controlled radical or group transfer polymerization.
According to the present invention, the silicon-containing vinyl copolymer includes three repeating units, that is, a maleic anhydride repeating unit represented by formula (I), a norbornene repeating unit represented by formula (II), and a vinyl repeating unit represented by formula (III). The norbornene repeating unit (II) includes an acid-labile group D, and the vinyl repeating unit (III) includes a silicon-containing group R2. Different from the conventional silicon-containing polymer, the silicon-containing group of the present invention, R2, is introduced into the vinyl repeating unit, rather than a norbornene repeating unit. In the silicon-containing vinyl copolymer of the present invention, the mole fractions of the repeating units (I), (II) , and (III) are 10-50:10-50:10-50, preferably 30-50:20-40:20-40.
According to a preferred example of the present invention, R2 can be 
wherein n is 0 or an integer of from 1 to 6, and R3 is C1-8 linear or branched alkyl. Preferably, n is 0 or an integer of from 1 to 3. Most preferably, n is 3, R3 is methyl, thus, R2 is 
According to another preferred example of the present invention, R2 can be 
wherein n is 0 or an integer of from 1 to 6, and R3 is C1-8 linear or branched alkyl. Preferably, n=0, and R3 is methyl, thus, R2 is 
In the norbornene repeating unit (formula (II)), D is an acid-labile protective group, which decomposes in the presence of an acid, so as to make the silicon-containing vinyl copolymer alkali-soluble. Representative examples of D include: 
wherein R4 is selected from the group consisting of hydrogen, C1-20 linear and branched alkyl, C3-20 cyclic and pericyclic alkyl.
The silicon-containing vinyl copolymer of the present invention has a weight average molecular weight of 3000 to 100000, is soluble in an organic solvent, and is suitable for use as a resin in thin film coating. The silicon-containing vinyl copolymer of the present invention can be used with a photoacid generator (PAG) to form a resist composition. The photoacid generator is preferably present in an amount of from 0.03 to 20% by weight of the silicon-containing vinyl copolymer.
Any suitable photoacid generator may be used. Preferred photoacid generators include, but are not limited to, triarylsulfonium salts, diaryliodonium salts, sulfonates, and mixtures thereof. Representative examples of the triarylsulfonium salts include triphenyltriflate, triphenylantimonate, methoxytriphenyltriflate, methoxytriphenylantimonate, trimethyltriphenyltriflate, and naphthalenetriflate. Representative examples of the diaryliodonium salts include diphenyliodoniumtriflate, di-t-butylbisphenyl-antimonate, and di-t-butylbisphenyltriflate.
The resist composition of the present invention is photosensitive at a wavelength of 100 nm to 300 nm, preferably at a wavelength of 193 nm or 248 nm.
The resist composition of the present invention can be used as a top layer in a bilayer resist system. First, a resin underlayer is coated on a silicon wafer and then cured by baking. Next, the resin composition of the present invention, which includes the silicon-containing vinyl copolymer, is coated onto the silicon wafer with the resin underlayer and then baked to form a top layer.
The wafer, which has been coated with the resin underlayer and the silicon-containing top resist layer, is exposed to radiation through a mask and then baked. Upon exposure, the labile group D in the silicon-containing vinyl copolymer of the present invention will be removed. Therefore, the exposed region of the top layer can be removed by developing the wafer with a conventional developer. Thus, a pattern of the top layer can be formed.
The patterned wafer is then subjected to oxygen plasma etching. In the unexposed region, silicon contained in the surface of the top layer will react with oxygen plasma to form nonvolatile products (i.e., SiO2). In the exposed region, the underlayer will react with oxygen plasma to form volatile products and will be readily evaporated by etching. Therefore, the pattern of the silicon-containing resist top layer can be transferred to the resin underlayer accordingly.
With regard to the resin underlayer, the resin suitable for use in the present invention can be phenolic resins, particularly novolak resins, such as formaldehyde cresol or formaldehyde phenol novolaks, polyimide resins, poly(meth)acrylate resins and styrene-allyl alcohol copolymer resins.
By means of the maleic anhydride repeating unit, the adhesion of the silicon-containing vinyl copolymer of the present invention to the resin underlayer can be increased. By means of both the maleic anhydride and norbornene repeating units, the degree of freedom of the copolymer backbone can be decreased, which will make the copolymer have a higher glass transition temperature (Tg). In addition, film cracking problem can also be solved.
The following examples are intended to illustrate the process and the advantages of the present invention more fully without limiting its scope, since numerous modifications and variations will be apparent to those skilled in the art.